Moodle performance

Moodle Performance

Overview

Moodle bottlenecks: too many DB queries per request, full-table scans, loading large recordsets into memory, blocking work on the request path. Mitigations: MUC caching, indexes, recordsets, ad-hoc tasks, careful capability checks.

When to Use

• Page renders slowly / hits per-page query limits
• N+1 query pattern in loops
• Memory-blow on large datasets
• Long-running operations on POST handlers
• Tuning a custom report or scheduled task

Diagnosis

Performance debug

config.php:

$CFG->debug = (E_ALL | E_STRICT);
$CFG->debugdisplay = 1;
$CFG->perfdebug = 15;     // shows query count + timings in footer

Site admin > Development > Debugging > Performance info: enables the footer toolbar (queries, time, memory, MUC hits/misses, sessions reads/writes).

$PERF

global $PERF;
$start = microtime(true);
// ... work ...
$PERF->dbqueries++;       // your manual counter
mtrace('Took ' . round(microtime(true) - $start, 3) . 's');

Slow query log

Postgres / MySQL slow-query log — turn on in dev. Moodle prefixes queries with the source class via $CFG->dboptions['debug'].

MUC — Moodle Universal Cache

3 cache modes:

Mode	Storage	Lifetime	Use
MODE_APPLICATION	Persistent (file/Redis/Memcached)	Until invalidated	Shared computed values
MODE_SESSION	Session	Session lifetime	Per-user computed values
MODE_REQUEST	PHP request	Single request	Memoize within request

Define a cache

db/caches.php:

<?php
defined('MOODLE_INTERNAL') || die();

$definitions = [
    'active_announcements' => [
        'mode'         => cache_store::MODE_APPLICATION,
        'simplekeys'   => true,
        'simpledata'   => true,
        'ttl'          => 600,
        'invalidationevents' => ['changesin_local_announcements'],
    ],
    'user_dashboard' => [
        'mode'       => cache_store::MODE_SESSION,
        'simplekeys' => true,
    ],
];

Bump version.php after edits.

Use the cache

$cache = \cache::make('local_announcements', 'active_announcements');
$value = $cache->get('all');
if ($value === false) {
    $value = $this->compute_announcements();
    $cache->set('all', $value);
}
return $value;

Invalidate

\cache_helper::invalidate_by_event('changesin_local_announcements', ['all']);
// or
$cache->delete('all');
$cache->purge();

Trigger invalidation from a settings save or an observer.

simplekeys / simpledata

• simplekeys: true — keys are alphanum (skips hashing) — faster
• simpledata: true — values are scalars/arrays of scalars (skips serialization) — much faster

Use both whenever possible.

Static cache (per-request)

$cache = \cache::make('local_announcements', 'request_lookup');
// MODE_REQUEST in caches.php — auto-cleared at end of request

DB optimization

Recordsets for large data

// BAD — loads everything into memory
$rows = $DB->get_records('huge_table');
foreach ($rows as $r) { /* ... */ }

// GOOD — streams
$rs = $DB->get_recordset('huge_table');
foreach ($rs as $r) { /* ... */ }
$rs->close();      // ALWAYS close

Use get_recordset_sql with LIMIT + offset for batched processing of millions of rows.

Avoid N+1

// BAD
foreach ($courses as $c) {
    $teacher = $DB->get_record('user', ['id' => $c->teacherid]);
}

// GOOD — single query
$tids = array_column($courses, 'teacherid');
[$insql, $params] = $DB->get_in_or_equal($tids);
$teachers = $DB->get_records_sql("SELECT * FROM {user} WHERE id $insql", $params);

Indexes

<INDEX NAME="userid-courseid" UNIQUE="false" FIELDS="userid, courseid"/>

Edit via XMLDB editor. Composite index column order matters — most-selective first, leftmost-prefix usable for partial queries.

EXPLAIN your queries

mysql> EXPLAIN SELECT ... ;
postgres=# EXPLAIN ANALYZE SELECT ... ;

Look for:

• type: ALL (full scan) — needs index
• Using filesort / Using temporary — sort spilling
• High rows estimate — selectivity issue

Move work off the request

Ad-hoc task — fire-and-forget

// classes/task/send_report.php
namespace local_example\task;
class send_report extends \core\task\adhoc_task {
    public function execute(): void {
        $data = $this->get_custom_data();
        // do work
    }
}

// trigger:
$task = new \local_example\task\send_report();
$task->set_custom_data(['userid' => $user->id, 'reportid' => $r->id]);
$task->set_userid($user->id);     // runs as that user
\core\task\manager::queue_adhoc_task($task);

Tasks run via cron. Long jobs don't block HTTP request.

Scheduled task — recurring

db/tasks.php:

$tasks = [[
    'classname' => 'local_example\task\cleanup',
    'blocking'  => 0,
    'minute'    => '0',
    'hour'      => '3',
    'day'       => '*',
    'month'     => '*',
    'dayofweek' => '*',
]];

class cleanup extends \core\task\scheduled_task {
    public function get_name(): string {
        return get_string('task:cleanup', 'local_example');
    }
    public function execute(): void {
        // ...
    }
}

Run cron: php admin/cli/cron.php. Production: cron entry every minute.

Locks for non-overlapping execution

$locktype = 'local_example_cleanup';
$lockfactory = \core\lock\lock_config::get_lock_factory($locktype);
if (!$lock = $lockfactory->get_lock('main', 5)) {
    return;     // another instance running
}
try {
    // ...
} finally {
    $lock->release();
}

Capability check optimization

has_capability() is expensive at scale. For lists:

// BAD — N capability checks
foreach ($users as $u) {
    if (has_capability('mod/quiz:attempt', $context, $u)) { /* ... */ }
}

// GOOD — single batched query
$users = get_users_by_capability($context, 'mod/quiz:attempt', 'u.id, u.firstname, u.lastname');

Output / page

$PAGE->set_pagelayout('embedded');     // skips heavy regions when appropriate
$PAGE->add_body_class('skip-some-blocks');

Lazy-load AMD modules:

// only load when needed
const onClick = async () => {
    const {init} = await import('local_example/heavy');
    init();
};

OPcache + APCu

php.ini:

opcache.enable = 1
opcache.memory_consumption = 256
opcache.max_accelerated_files = 20000
opcache.validate_timestamps = 0    # production only
opcache.revalidate_freq = 0

Moodle benefits massively from OPcache. With validate_timestamps=0, restart PHP after deploys.

APCu for Moodle config cache: $CFG->localcachedir = '/var/cache/moodle';

Sessions

• Use Redis sessions in production ($CFG->session_handler_class = '\core\session\redis')
• File sessions don't scale past one app server
• Session writes — see "session lock" issues if multiple AJAX requests block on session

// release session lock early when only reading
\core\session\manager::write_close();

Frontend

• Run npx grunt amd — production builds are minified
• Enable browser caching: $CFG->cachejs = true; $CFG->cachetemplates = true;
• Theme designer mode ($CFG->themedesignermode) — OFF in production (disables CSS cache)

Common Mistakes

Mistake	Fix
get_records on huge table	get_recordset + close
N+1 in loops	Batch with get_in_or_equal
Sending email on POST	Move to ad-hoc task
has_capability per row	get_users_by_capability
MUC without simplekeys/simpledata	Set both true when possible
MUC MODE_REQUEST for cross-request data	Use MODE_APPLICATION
Long lock without try/finally	Release in finally block always
Theme designer mode on prod	Off — kills CSS caching
validate_timestamps = 1	OFF in prod, restart on deploy
Forgetting $rs->close()	Recordset leaks — always close
File sessions on multi-app-server	Switch to Redis/Memcached

Profiling

xhprof / tideways:

// quick on-demand profile
$CFG->profilingenabled = true;
$CFG->profilingautostart = false;
// add ?PROFILEME to URL — view in admin/tool/profiling

Per-request: Site admin > Development > Profiling.

References

• MUC: https://moodledev.io/docs/apis/subsystems/muc
• Tasks API: https://moodledev.io/docs/apis/core/task
• Performance recommendations: https://docs.moodle.org/en/Performance_recommendations
• DB API recordsets: https://moodledev.io/docs/apis/core/dml#get_recordset
• Profiling: https://moodledev.io/general/development/tools/profiling